A Resonant Solution for Fast, Flexible EV Charging

A new high-voltage booster enables fast charging of 800V vehicles even at 400V charging stations.

The DC/DC multi-converter can transform 800V into 48V, 12V and 400V, and may enable the separate traditional 12-V battery in EVs to be eliminated. (Preh)

The infrastructure of 400V charging stations has been massively expanded in Europe, with some 400 locations in place and more on the way. However, 800V vehicles currently are left out because their batteries cannot be easily recharged at 400V stations. This situation has prompted Preh’s development of a new high-voltage booster.

Dr. of Eng. Joachim Wagner is head of pre-development e-mobility at Preh.

The onboard booster enables 800V vehicles to be fast charged at conventional 400V stations. The 400 volts supplied is "boosted" to 800 V with an output of up to 150 kW. The result is a vehicle battery charged from 5% state of charge (SoC) to up to 80% SoC in just more than 35 minutes, while significantly expanding charging opportunities for high-voltage vehicles.

The device is a classic resonant converter, as typically used in integrated circuits. It is available in two power classes, 50 kW and 150 kW. Its special topology enables the booster to achieve an efficiency of over 97%. Achieving such efficiency required the development and testing of suitable software algorithms for the interoperable use of all DC charging stations available worldwide.

Around the world, country-specific standards for fast-charging stations vary, as do the charging and communication protocols related to the Combined Charging System (CCS), CHAdeMo or GB/T standard. In addition, however, the topics of charging-cable precharging, insulation monitoring or PE plausibility and balancing vary, depending on the geographic location and require the use of intelligent software algorithms. Preh offers a globally interoperable solution for this with generic software and the same hardware. Both have been tested in field trials at all common fast charging stations.

Three-stage DC/DC conversion

The booster architecture has two additional advantages: First, by means of a 150-kW booster at fast-charging stations limited to a maximum of 750V (for example, in China), the vehicle’s 800-V propulsion battery can be fully charged to 100% without any loss of time due to switching. Second, the vehicle's electrical system components are supplied with voltage without interruption during the charging process because the 800V can be maintained stably.

A separate DC/DC converter, mounted on top of the booster, serves as a “multi converter” capable of transforming the 800V into three different voltages: 48V, 12V and 400V. For example, the 48V voltage might power a high-performance vehicle’s roll-stabilization feature. The system also works bidirectionally; a recuperation function converts the 48V back into 800V. The 12V voltage would power the vehicle’s electrical hotel loads and the 400V voltage would operate the electric A/C compressor. Some OEMs already are examining ways to replace the traditional 12V battery in EVs, particularly to reduce weight. In the future, this may be feasible using a smart system architecture that includes a combination of DC/DC converters.

The three-stage DC/DC converter features a package-efficient design. Since small installation spaces leave minimal room for electromagnetic compatibility (EMC) isolation measures, one of the challenges was to minimize the occurrence of EMC interference. Therefore, resonant converters are used per stage, which enable soft and loss-free voltage conversion at an efficiency of up to 96%.

This high efficiency decisively reduces the resulting waste heat and enables a compact system design. The selected topology keeps the internal voltages below 650V, which means that cost-effective Si MOSFETs (Silicon metal-oxide semiconductor field-effect transistor) in surface mount device (SMD) design can be used. Also, the amplitude of the ripple current can be kept small.

Dr. Joachim Wagner said Preh’s next-gen EV booster now in development is a modular design. (Preh)

At the heart of the converter is a powerful computing unit that controls the entire multi-converter system using an intelligent control concept. Both the multi-converter and the high-voltage booster fit into a compact installation space and can be integrated into the electric drivetrain in a space-saving manner. The reliability of such components is ensured at Preh in its own 250-kW high-performance test laboratory. Here, technicians can carry out environmental impact simulations as well as mechanical, electrical and service life tests at its headquarters in Bad Neustadt, Germany.

What’s next in converters

The next generation of the high-voltage booster currently is under development. It uses a modular platform concept with common parts. In the next generation of the DC/DC multi-converter, development is focusing on further increasing power density while further optimizing installation space. Also under further development is the battery management system, which in its current version is in series production and is on the road in various hybrid vehicles and in industrial trucks.

Future milestones also are expected to include an 11-kW onboard charger solution capable of providing bidirectional charging options for vehicle-to-grid (V2G) and vehicle-to-load (V2L), as well as the further development of wireless charging solutions at 11 kW.

Joachim Wagner is head of pre-development e-mobility at Preh.